Hello, again. Welcome to this tutorial on the internal
anatomy of the spinal cord. Once again, we're confronted with the
complexity of the nervous system. So, our core concept is that the brain is
the body's most complex organ. And our learning objective for today is
that I want you to be able to identify the major subdivisions of the spinal cord as
seen in representative transverse cross-sections.
So, this requires enough familiarity to the internal anatomy of the spinal cord
that we can view in cross-sections so that you can understand what level of the
spinal cord is in view and what are the important gray matter and white matter
structures that we can find there. Alright.
Well, let's begin by looking at a histological section of the spinal cord.
This happens to be from the lumbar enlargement of the human spinal cord.
And this is a section that's been stained for the presence of Nissl substance.
Nissl is a term we use in neurohistology. It refers to rough endoplasmic reticulum.
So, wherever we have rough ER within a cell, which is typically in the cytoplasm,
and there's some rough ER within the nucleus as well.
But mainly in the cytoplasm and perhaps into the proximal dendrites, that's where
we will find the stains. So, this is going to be a stain for cell
bodies, and for neurons, for also the proximal portions of the dendrites.
So what's not stained here are myelinated axons.
So, this is one way of looking at the nervous system that highlights the
presence of cell bodies. And what we can see in this image is that
within this spinal cord, we have gray matter that is central to the cord and
that gray matter is surrounded by white matter.
So white matter is found all in these peripheral regions on both sides of the
spinal cord. Okay.
Now let me give you some terms that will help you as we go and look at the
structure within the spinal cord. So, the white matter that we find on the
dorsal aspect, and this is dorsal, is called the dorsal column.
The white matter that we find on the lateral sides of the cord, is called the
lateral column. And the white matter that we find in the
ventral or the interior parts of the cord, is called the ventral column.
So, this word, column, appears repeatedly in our lexicon that we use to describe
white matter in the spinal cord. So, when you see the word, column, think
white matter. Now, occasionally, we apply the word,
column, to talk about the organization of cell bodies but I'll try to be very clear
when that's the case. Otherwise, when you see the word column in
your readings or in our discussions, think about white matter.
Okay. So now, let's turn our attention to the
gray matter of the spinal cord and we can recognize basically two broad regions of
gray matter in the spinal cord. There's something that we call the Dorsal
horn, which can be found in this aspect of the grey matter of the spinal cord.
And then, there is a Ventral horn. And the Ventral horn, then, would be
pretty much the rest of this, excluding, perhaps, what we might consider to be an
intermediate zone between the Dorsal horn and the Ventral horn.
We'll come back and talk about that as we get into different levels of the spinal
cord. Bu the Dorsal horn and the Ventral horn
provide us with our basic dichotomy between a sensory zone of gray matter,
which is what we find in the Dorsal horn, and a motor zone, which is what we find in
the Ventral horn. So, in the Dorsal horn, we have
longitudinal layers of cells that are organized along the long axis of the
spinal cord one layer on top of the other almost like a long series of strips of
cells. And these strips have been recognized by
anatomists over the years, and the scheme that seems to have, have stuck the best
was proposed by an anatomist by the name of Rexed.
And we call this scheme Rexed's layers or Rexed's laminae of the Dorsal horn.
So, that's what the roman numerals are over here to the left side.
So, the Dorsal horn is essentially laminae 1 through 6.
We'll spend some time talking about these laminae when we talk about mechanisms of
pain and temperature processing, especially lamina number 2, also known as
the substantia gelatinosa or the gelatinous layer of the Dorsal horn.
So, we'll come back to that in a later session.
For now, I just want you to get the idea that the Dorsal horn is gray matter that
is receiving incoming sensory signals from nerves that are running in through the
Dorsal roots and synapsing upon cells in the Dorsal horn.
Now, let's turn our attention to the Ventral horn, which is where we find our
lower motor neurons, that is, the motor neurons that innervate muscle, and the
local circuit neurons that coordinate the output of those lower motor neurons.
And here, we have less of a layered organization to these cells, more of a
column organization of the cells. So, for example, we may find a column of
cells that all innervate the same muscle. Not necessarily the same muscle fiber but
perhaps a distributed set of muscle fibers that are within the same anatomical unit
that we would recognize as a muscle. So, if the Dorsal horn is organized into
long strips of cells, then the Ventral horn is organized more into columns.
And so here in lamina 9, we actually have something more like a rod-shape of cells.
And there are other more rod-shapes collections of cells that we recognize as
laminae 7 through 9. Now, we still use the term laminae, but as
I'm suggesting, the organization in the ventral horn is more columnar or
rod-shaped, than it is laminar. Alright.
So there are connections between the Dorsal horn and the Ventral horn made by
local circuit interneurons, and in one special case, there is an incoming sensory
axon that synapses directly on alpha motor neurons that mediates our very fast
myotactic reflexes. So, we'll talk about that in a later
tutorial. For now, I just want you to have an
overview of the organization of the gray matter in the spinal cord.
And since it's gray matter, we expect to see cell bodies and that's why I'm showing
you this Nissl stain. But now, let's turn our attention to the
white matter that surrounds that gray matter.
And for that purpose, I want to show you a myelin stain section.
So, so this is a typical way that a myelin stain section might look at a
cross-section through the nervous system. One can often see a great detail that is
otherwise not made visible in Nissl stain if you stain with silver salts or perhaps
gold chloride or otherwise make the white matter look dark.
So, please don't be thrown off by this convention.
Essentially, wherever we see dark staining, we are looking at white matter.
So, that means that where we see the lighter stain, that is going to be gray
matter. So, this is essentially the inverse visual
appearance of what we just looked at with the Nissl stain.
Okay, I think that should be clear enough. So, what we have here is white matter that
surrounds gray matter. And we recognize, basically, three zones
of white matter around this gray matter. On the dorsal side of the spinal cord, or
the posterior side, we have the dorsal column of white matter.
On the lateral side, roughly to about this region, we have what we call the lateral
column. And then, on the ventral or the anterior
side, we have white matter that we recognize as the ventral or the anterior
column of white matter. Now, in each region of white matter, we're
going to find different kinds of sensory or motor pathways.
And we'll get into that in quite a bit of detail as we progress over the next couple
of weeks. So, I won't give you all that detail now
except to just make a few broad statements about how these tracks are organised
within this white matter. Essentially, the dorsal column is a
sensory zone. There are sensory axons that are ascending
the spinal cord in the dorsal columns. In the lateral column, we have a mixture
of sensory axons, and one very large and very important motor pathway that occupies
actually most of this dorsal, excuse me, most of this lateral column, and that's
the lateral cortical spinal tract. We'll spend a fair amount of time on the
organization of that one pathway. Now, likewise on the ventral or the
anterior column, we have a mixture of both sensory and motor pathways.
And here, with respect to the sensory pathway, there is one very important
pathway that we'll spend a fair amount of time talking about.
And that pathway we find roughly in this region of the ventral or anterior column.
And this is where we find our pain pathways that are ascending the spinal
cord, conveying sensations about pain and temperature to processing stations in the
brain stem and in the forebrain. Much of the rest of this system is a
collection of small descending motor pathways that are conveying signals mainly
from the brain stem to the ventral horn of the spinal cord.
